The aim of this project is to develop a method for serotyping certain chlamydiae (bedsoniae). This genus, which includes the agents of psittacosis, lymphogranuloma venereum, and trachoma, is currently divided into two species based on inclusion type and sulfonamide sensitivity. A plaque assay has already been developed to measure infectivity of many bedsonial isolates, and reduction in plaque count has been observed when hyperimmune antisera is premixed with the inoculum. Preliminary studies have proven the potential for application of this test in serotyping isolates. Systematic surveys will be made of bedsonial isolates, using the plaque-reduction test to clarify their antigenic relationships. This technique, in addition to its taxonomic use, may provide a valuable tool for epidemiological studies.